We briefly describe the soft X-ray image of M87 obtained using the imaging proportional counter aboard the Einstein Observatory. These data provide further strong evidence for the existence of a massive halo of dark matter surrounding M87 and allow a much more precise determination of its mass. Two pointing positions of the satellite were analyzed; one centered on M87, the other 63’ south and 25’ east of M87. The field of view of the imaging proportional counter is 60’ × 60’, and it attains a two dimensional spatial resolution of ˜ 1.5’ in a spectral range spanning 0.1 to 4.5 keV.
As previous reports had suggested (Gorenstein et al. 1977; Fabricant et al. 1978), the present observations show M87 to be a strong, very extended, thermal X-ray source with a temperature near 2 keV, surrounded by weaker and still more extended emission from hotter gas associated with the Virgo cluster as a whole (Davison, 1978; Lawrence, 1978). We find M87 to have a total 0.5-4.5 keV X-ray luminosity of about 2 × 1043 ergs/sec, and an extrapolated 2-6 keV luminosity of approximately 1 × 1043 ergs/sec. The total mass of gas inferred from the X-ray measurement exceeds 1012 solar masses.